The compositional dependence of the crystalline phase and properties of precipitates (Ti (x) Sn1-x O-2) in the TiO2-SnO2 system, which were hydrothermally formed at 100-200 A degrees C from the precursor solutions of TiCl4 and SnCl4 under weakly basic conditions in the presence of tetramethylammonium hydroxide (TMAH) was investigated. Rutile-type (Ti, Sn)O-2 solid solutions with nano-sized crystallite were directly formed at 180 A degrees C in the composition range of x = 0-0.8. Nanoparticles with anatase crystallite around 10 nm as a main crystalline phase of precipitates that were formed in the compositions x = 0.9 and 1.0 showed similar photocatalytic activity. As the hydrothermal treatment temperature rose from 100 to 200 A degrees C, the crystallite size of rutile solid solution, Ti0.5Sn0.5O2, increased from 2.5 to 8.0 nm. The optical band gap of the samples changed in the range of 2.93-3.25 eV depending on their composition in the system. At the composition of x = 1.0, submicron-sized anatase-type pure TiO2 particles (sizes of cuboid sides are around 100-120 nm) with pretty high crystallinity and superior photocatalytic activity were formed from the aqueous solution of TiCl4 under basic hydrothermal condition at 180 A degrees C in the presence of TMAH with concentration as 1.3 times high as the condition in the case of the nano-sized anatase.